Aquaculture production systems in developing countries are largely based on the use of unimproved species and strains. As knowledge and experience are accumulated in relation to the management, feeding and animal health issues of such production systems, the availability of genetically more productive stock becomes imperative in order to more effectively use resources. For instance, there is little point in providing ideal water conditions and optimum feed quality to fish that do not have the potential to grow faster and to be harvested on time, providing a product of the desired quality. Refinements in the production system and improvement of the stock used must progress hand in hand. There is documented evidence about the success of selective breeding in several species, and there are indications that the potential benefits are very large. However, it is quite clear that such gains will have no impact on farmers unless the progeny from improved strains reach the production system in a state that makes them capable of prospering during the grow-out period until they reach market weight. Achieving genetic change in a population will often be easier than achieving effective multiplication and dissemination of the resulting improved strain. In the former case, we simply have to control the reproduction and mating among the fish, whereas in the latter we have to influence people and the way they operate. Assuming that because we have an improved strain, hatchery managers will effectively multiply it and disseminate it, and that farmers will adopt it, is unrealistic. In this paper we deal separately with genetic and non-genetic issues pertaining to the multiplication and dissemination of improved strains. The separation is somewhat arbitrary, and as will be evident from our discussion, there is frequent interaction between the two.